Looking north along Maryland's Kent Island toward the Bay Bridge, you can see a diversity of land uses. Excess nutrients and sediment washing off the Chesapeake Bay's watershed are causing harm to the estuary's ecosystem. Scientists are using computer simulations to quantify these causes and effects to help leaders find the right solutions. Photograph, David Harp

SINCE THE 1980s, A FEDERAL-STATE partnership has labored to restore the nation's largest estuary. Known as the Chesapeake Bay Program, it is managed by the Environmental Protection Agency (EPA) and has more than 100 partners, including the six states and the District of Columbia whose waters drain into the Bay. All share an urgent goal: to improve the water quality of the Chesapeake Bay and restore the estuary's health to a sustainable level.

Now the Chesapeake Bay Program has reached an important milestone. The program is conducting a "midpoint assessment," a review of the Chesapeake Bay Model, the key tool the program uses to set cleanup goals for the rivers and mainstem of the Bay and to measure progress in achieving them. This midpoint assessment lies halfway between two other important dates — one past (2010), one to come (2025).

In 2010, in a new push to restore the Bay, the Chesapeake Bay Program adopted unprecedented mandatory limits on how much excess sediment and nutrient runoff could enter the estuary without exceeding water quality standards. The limits are called the total maximum daily load (TMDL) or "pollution diet." The TMDL is designed to remedy problems like the low-oxygen "dead zones" that plague the Chesapeake. The Bay's TMDL is the largest geographically and most complex in EPA history.

The EPA requires the six states and the District of Columbia to have in place all measures needed to achieve their pollution diets by 2025. Collectively, they must cut a quarter of the nitrogen and phosphorus entering the Chesapeake Bay and a fifth of the sediment coming in, compared with 2010 levels.

The midpoint assessment offers leaders the chance to make course corrections to achieve the 2025 goals. They may need to revise the region's watershed implementation plans — detailed strategies for reducing sediment and nutrient pollution, such as changing farming techniques and reducing the amount of polluted stormwater runoff.

And why does anyone believe these actions stand a chance of restoring the Bay? Because the Chesapeake Bay model says they will.

The Bay model is actually a set of computer simulations that together mimic the natural processes that sustain the estuary's ecosystem. The model was used to set the pollution diet; now scientists are working to incorporate the best science and data to improve the model as part of the midpoint assessment (see A Bay in a Box).

As scientists work to review the Bay model, they are also looking beyond the 2025 horizon. Where is the model headed next? According to Carl Cerco, the long-time lead modeler for EPA's Chesapeake Bay Program, the concepts underlying the model were developed during the mid-1980s. New research could now bring fundamental changes in the model's very foundations. It's about time, says Cerco. "Let's move on."

Note from the editors: The future of the Bay model and the entire Bay restoration effort recently came into question when the White House proposed to Congress the elimination of all funding for the Chesapeake Bay Program and for the National Sea Grant College Program, including Maryland Sea Grant, the research, education, and outreach program which publishes Chesapeake Quarterly. It is up to Congress, though, to set spending levels in appropriations bills.